In a circuit employing an input/output (I/O) buffer (also known as line driver), the buffer is typically put on a transmission line whose characteristic impedance is unknown, but known to lie in the range of, for example, 30-120 ohms. The far-end of the line is capacitively loaded with the value of the load capacitor, which is typically bounded within the range of, for example, 5 pF-50 pF. As the loads at both ends are not matched with the transmission line, this will result in reflection occurring at both ends of the line, thereby distorting the signals communicated over it. The memory in the channel in the form of previous bits reflected from both ends can cause a reduced eye opening and high jitter at the receiving end, thereby causing an increased bit error rate (BER). For example, in most high-speed applications for the design of serial links, the termination impedance at the receiver end of the transceiver system has to be matched to the transmission line's impedance to control the reflections and jitter at gigabits per second. As the system operates at very high speeds, it is very sensitive to sub-pico-second jitters and other reflections that occur at both ends of the transmission line. Therefore, it is often very important to calibrate this termination impedance equal to the unknown impedance of the transmission line with high accuracy.
There have been attempts at solving the above problems. For example, in one method that has been used in some video-line drivers, the calibration has been proposed in the analog domain, whereby the current of the buffer is set depending upon the current division among the buffer and the line. In another proposed method, handshaking provides the necessary information from the receiver's end on the same channel, which gives the information about the eye closure of the eye; then equalization is done at the transmitter's end.